1. Field of the Invention
The present invention relates to papermaking, and, more particularly, to a structured fabric employed in papermaking.
2. Description of the Related Art
In a conventional papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper “stock”) is fed into a gap between two endless woven wires that travel between two or more rolls. At least one of the wires is often referred to as a “structured fabric” that provides a papermaking surface on the upper surface of its upper run which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium, thereby forming a wet paper web. The aqueous medium drains through mesh openings of the structured fabric, known as drainage holes, by gravity or vacuum located on the lower surface of the upper run (i.e., the “machine side”) of the fabric.
After leaving the forming section, the paper web is transferred to a press section of the paper machine, where it is passed through the nips of one or more pairs of pressure rollers covered with another fabric, typically referred to as a “press felt.” Pressure from the rollers removes additional moisture from the web; the moisture removal is often enhanced by the presence of a “batt” layer of the press felt. The paper is then transferred to a dryer section for further moisture removal. After drying, the paper is ready for secondary processing and packaging.
Typically, papermakers' fabrics are manufactured as endless belts by one of two basic weaving techniques. In the first of these techniques, fabrics are flat woven by a flat weaving process, with their ends being joined to form an endless belt by any one of a number of well-known joining methods, such as dismantling and reweaving the ends together (commonly known as splicing), or sewing on a pin-seamable flap or a special foldback on each end, then reweaving these into pin-seamable loops. A number of auto-joining machines are available, which for certain fabrics may be used to automate at least part of the joining process. In a flat woven papermakers' fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross machine direction.
In the second basic weaving technique, fabrics are woven directly in the form of a continuous belt with an endless weaving process. In the endless weaving process, the warp yarns extend in the cross machine direction and the filling yarns extend in the machine direction. Both weaving methods described hereinabove are well known in the art, and the term “endless belt” as used herein refers to belts made by either method.
Effective sheet and fiber support are important considerations in papermaking, especially for the forming section of the papermaking machine, where the wet web is initially formed. Additionally, the structured fabrics should exhibit good stability when they are run at high speeds on the papermaking machines, and preferably are highly permeable to reduce the amount of water retained in the web when it is transferred to the press section of the paper machine. In both tissue and fine paper applications (i.e., paper for use in quality printing, carbonizing, cigarettes, electrical condensers, and the like) the papermaking surface comprises a very finely woven or fine wire mesh structure.
In a conventional tissue forming machine, the sheet is formed flat. At the press section, 100% of the sheet is pressed and compacted to reach the necessary dryness and the sheet is further dried on a Yankee and hood section. The sheet is then creped and wound-up, thereby producing a flat sheet.
In an ATMOS™ system, a sheet is formed on a structured or molding fabric and the sheet is further sandwiched between the structured or molding fabric and a dewatering fabric. The sheet is dewatered through the dewatering fabric and opposite the molding fabric. The dewatering takes place with airflow and mechanical pressure. The mechanical pressure is created by a permeable belt and the direction of air flow is from the permeable belt to the dewatering fabric. This can occur when the sandwich passes through an extended pressure nip formed by a vacuum roll and the permeable belt. The sheet is then transferred to a Yankee by a press nip. Only about 25% of the sheet is slightly pressed by the Yankee while approximately 75% of the sheet remains unpressed for quality. The sheet is dried by a Yankee/Hood dryer arrangement and then dry creped. In the ATMOS™ system, one and the same structured fabric is used to carry the sheet from the headbox to the Yankee dryer. Using the ATMOS™ system, the sheet reaches between about 35 to 38% dryness after the ATMOS™ roll, which is almost the same dryness as a conventional press section. However, this advantageously occurs with almost 40 times lower nip pressure and without compacting and destroying sheet quality. Furthermore, a big advantage of the ATMOS™ system is that it utilizes a permeable belt which is highly tensioned, e.g., about 60 kN/m. This belt enhances the contact points and intimacy for maximum vacuum dewatering. Additionally, the belt nip is more than 20 times longer than a conventional press and utilizes airflow through the nip, which is not the case on a conventional press system.
Actual results from trials using an ATMOS™ system have shown that the caliper and bulk of the sheet is 30% higher than the conventional through-air drying (TAD) formed towel fabrics. Absorbency capacity is also 30% higher than with conventional TAD formed towel fabrics. The results are the same whether one uses 100% virgin pulp up to 100% recycled pulp. Sheets can be produced with basis weight ratios of between 14 to 40 g/m2. The ATMOS™ system also provides excellent sheet transfer to the Yankee working at 33 to 37% dryness. A key aspect of the ATMOS™ system is that it forms the sheet on the molding fabric and the same molding fabric carries the sheet from the headbox to the Yankee dryer. This produces a sheet with a uniform and defined pore size for maximum absorbency capacity.
U.S. Patent Application Publication No. to QUIGLEY, the disclosure of which is hereby expressly incorporated by reference herein in its entirety, discloses a fabric for making a bulky web. The fabric includes a machine-facing side and a web-facing side including pockets formed by machine direction yarns and cross-machine direction yarns, but more machine direction yarns than cross-machine direction yarns. Adjacent pockets are offset from each other and are defined by machine direction and cross-machine direction knuckles.
U.S. Pat. No. 5,429,686 to CHIU et al., the disclosure of which is hereby expressly incorporated by reference herein in its entirety, discloses a throughdrying fabric for the drying section of a papermaking machine. The fabric has a load-bearing layer and a sculpture layer. The sculpture layer is characterized by impression machine direction knuckles formed as warp knuckles floating over a plurality of shutes but positioned substantially above the tops of the lowest shute knuckles in the load-bearing layer so as to provide machine direction knuckles projecting in the sculpture layer.
International Publication No. WO 2006/113818 to KROLL et al., the disclosure of which is hereby expressly incorporated by reference herein in its entirety, discloses a through-air-drying fabric for producing tissue paper and related products on a papermaking machine includes a plurality of warp yarns interwoven with a plurality of weft yarns to produce a paper-side surface pattern characterized by alternating first pockets and second pockets. The first and second pockets are bounded by raised warp yarns and raised weft yarns produced by knuckles in the fabric pattern. The first pockets are preferably larger in area than the second pockets. The fabric base weave in the interior of the first pocket is preferably a plain weave pattern. The interior of the second pocket may also be bisected by a raised weft yarn.
What is needed in the art is an efficient effective fabric weave pattern to be used in a papermaking machine.